King, who caucuses with Democrats, joined Shaheen in Concord on Friday hours after he endorsed Maine Republican Sen. Susan Collins in her re-election bid. Before being elected to the Senate, both King and Shaheen served as governors of their respective states, overlapping for six years during which they worked together on numerous issues, including economic development. In the Senate, they were among a bipartisan group that helped end the government shutdown last fall.

Several Republicans are vying to challenge Shaheen, including former Sens. Bob Smith and Scott Brown, former state Sen. Jim Rubens and conservative activist Karen Testerman.